|
Hey Kevin,
Check your fuel FLOW RATE, especially at your
normal climb angle (on the ground). It sounds like it's losing fuel flow on take-off - the carb is
using whatever fuel is in the bowl, and then running lean. You
might have to jack the fuselage to the required angle in case there is something
in the tank that is occuding the pick-up(s). However, if you can
reproduce it on the ground doing a run-up, it definitely sounds like lack
of fuel FLOW (not pressure) to me. If the engine will maintain 1700
RPM, then it is getting almost but not quite enough fuel FLOW at
WOT.
Various things come to mind. FIrstly,
(and most easily), a partially blocked fuel filter (just because it's new
doesn't mean it'll work!!). So change that first. Use a different
brand to eliminate faulty batch manufacture.
I once had a mysterious problem with an RX4.
Chased it for weeks. Turned out to be a bee in the fuel line. (I'm serious
- the guy who owned the car used to work for me and is still around and will
verify this!!). Anyway, Jack's pride & joy would work just fine
around town. But out on the open road, it would run out of fuel in
3rd gear (usually when overtaking). Turned out that a bee (very dead at
this stage) had got into the fuel tank. (The problem occurred soon
after Jack filled up at some outback petrol station when he was on
holidays). The bee immediately got stuck in the fuel supply
line. At normal cruising flow rates, there was sufficient fuel to
run the car. On hard acceleration, the bee's wings &
body would PARTIALLY block the supply line in the tank. We only found
it when we removed the tank, replaced it with another one, and cut
the old one open. You just wouldn't read about it in Ripley's or the War
Cry!
I've also had pieces of fuel gum (and one instance
a piece of cardboard) block the fuel pick-up. When fuel sits a long time
at a bowser storage tank, especially if there is water at the bottom
of the tank, it will form a sheet of gum at the fuel/water
interface, almost like condom latex, (but nowhere near as
strong). This film of gum gets broken up the next time a tanker does
a dump, and bits of this will very effectively block a fuel pick-up,
or cripple a fuel filter.
I've seen it happen on several occasions over the
years. It was particularly prevalent in the early '70s during the
phase out of "standard" petrol before the introduction of unleaded into this
country (all rotaries used to run on standard not super - which was heavily
leaded back then). I found out the hard way soon after I had purchsed my
RX2 Coupe in 1971. Filled up at an "out of the way" petrol station (where the
fuel sales volume on standard was virtually nil), and immediately had
problems. After eventually getting back to Sydney, we quickly
diagnosed it as a tank problem. Even steam cleaning the tank didn't fix
it. The tank was replaced, and we cut open the old one, and
there was all these bits of gum, just like a thin membrane tyhat had been
cut up.
So the place I'd be looking is in your fuel SUPPLY
system, as it seems you have done everything else except check fuel FLOW
rate. Can't guarantee that this IS the problem, but the quick and
easy way would be to check your open flow rate at the carb, and then
compare it with somebody else's. Alternatively, you can work
backwards as you know that the engine requires at least 0.55 Lbs per hour
per BHP at WOT. (First convert that to litres or pints or quarts per minute
and you can direct measure it, or run the fuel into a can for 1 minute and
then go and weigh it). Fuel flow rate should be at least 25% more than the
max required at WOT so that your carb doesn't run dry.
One other area of the problem could be the
incorrect needle/seat arrangement on the carb, or a partially blocked
filter at the banjo - I'm not privvy to what carb you are running so this
is conjectural as I plead total ignorance of Lycoming carbs.
However, all stock Mazda carbs have fine brass gauze mesh
filters, both at the banjo, and on top of the needle&
seats.
Additionally, in the racing 13Bs, an IDA
Weber would run out of fuel as the float bowl was just way too small for
anything over 250 BHP. We used to replace the 2.5 needle/seat assys with
3.0 units. But we would STILL run out of fuel on long
straights. Extended float bowls were then added, which did fix
the problem, but welding the Weber material was tricky, especially
if it had been saturated with oil laced fuel for a long time (sometimes the
castings were slightly porous)..
My final solution was to add an EXTRA float
bowl (from an SU carb), with it's own needle & seat, and a fuel
return. We could then go back to a 2.5 Weber needle &
seat, as the 3.0 one was prone to flooding at idle and on the
over-run. (Nothing good has ever come of fuel dilition of engine oil in a
race engine). This fixed all fuel flow issues. Some time
later, affordable aftermarket EFI came along, and all this
passed into history. But fuel SUPPLY issues are even MORE critical with
EFI.
So these are just a few areas where you can
look. Be systematic, and start at the connection between the carb
and the supply line. You will then quickly eliminate either the supply to
the carb, or the carb needle & seat itself. If the open pipe
fuel flow is more than adequate, then I'd be looking inside the carb for
restrictions. As I have said, this is only a guide. Feel free
to contact me off line if you feel the need.
Cheers,
Leon
----- Original Message -----
Sent: Sunday, July 24, 2005 6:33 AM
Subject: [FlyRotary] Lycoming debugging
test - -HELP!
I realize that this isn't exactly the
right forum, but you guys seem to have excellent debugging skills. my
local RV group is totally worthless ("I find the carnauba wax doesn't show
fingerprints as much as the....")(except Mike!) the on-field mechanics
are sick of me coming by.
my O-320 RV-6A is
having RAG (rpm attention getter) problems. it runs fine until about
300' on take-off, then the rpm's drop to like 1700 (from 2200).
switching mags to electronic ignition(Rose) makes no difference. mixture
no diff. pulling the throttle it will seem to run fine at
1700rpm. I have 4 new cyls that I am attempting to break in.
ground run ups can only go like 2 minutes before CHT's exceed 400
degrees. I discovered I can get the rpm drop on the ground if I run it a
minute or so full power. cht's will typically approach 415 degrees when
the drop takes place. I don't know if the temp is significant or not, it
does seem that way. I have a brand new slick left mag. rose
checked the electronic ignition and could find nothing wrong, even put it in
an oven to test. I installed a new carb since the old one showed signs
of leakage (and I'm at whits end). no change. my fuel pressure has
been showing 8psi this last year, up from 6 psi when originally installed.(no
idea why) I installed a pressure regulator in-line which works like a
charm. I can dial whatever pressure I like. 2 psi makes no
difference. I bypassed the mechanical fuel pump and ran it just on the
facet electric. 2 psi, no difference. I discovered that you can run
the carb for maybe 30 secs with no fuel pressure while it burns off the bowl
gas. premier engines suggested induction leak making the engine run
lean. no leaks detected cold or hot engine. (pressurized intake with
vacuum, sprayed soapy water)
if the engine didn't
run right to start with then many explanations would hold. but it runs
fine for several minutes. it runs the same under mag or electronic
ignition both before and after the problem kicks in. everyone really
wants to blame the rose ignition, but I'm not seeing any connection.
with such a quick run-up my oil temp barely registers, so I doubt parts are
seizing. I have new mineral oil in it for breakin right now. there
is a bit of MMO in the fuel too I believe, left over from a mechanics "what
the hell" attempt from way back. my cyls are all new, rebuilts, so no
valve problems. I checked the push rod lengths when I reinstalled
them. the engine isn't missing when the problems occurs, it simply won't
run faster and it seems to prefer the throttle pulled back to match the rpm it
is putting out. I can't figure out any more lean mixture scenarios to
try. the new carb runs just like the old one did. I recently
replaced the mag harness with a much newer, used one. I tested it and it
was fine. the plugs are all new, but again, no diff between mag and elec
ignition. if I flip the ignition to R and shut off the elect ig then the
engine quits as would be expected, no wiring problems.
fuel, spark, air,
timing, that's all there is. this damn lycoming lawn mower has me
stumped. my engine has been running fine up until this (hey, we all have
"cracked" cylinders, get out your microscope and look!). you guys have
any ideas?
----- Original
|